The present invention relates to an imaging lens for forming an image on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a cellular phone, a digital still camera, a portable information terminal, a security camera, an onboard camera, and a network camera.
In these years, most cellular phone models include an onboard camera as standard equipment, and there have been attempts to increase added values of cellular phones. Furthermore, there are progresses in integrating a digital still camera to a cellular phone every year, and in these days, there have been available cellular phones, which have optical performances and various functions that are comparable to those of digital still cameras.
An imaging lens to be mounted in such cellular phone is strongly required to have sufficient optical performances suitable for resolution of an imaging element and a small size. Conventionally, both sufficient optical performances and small size have been attained by an imaging lens with a two-lens configuration or a three-lens configuration. However, optical performances on demands are higher and higher every year as an imaging element has higher resolution, and it is impossible to sufficiently correct aberrations by such two-lens or three-lens configuration, so that it is difficult to fully satisfy the demanded optical performances.
For this reason, there are studies on adding another lens, i.e., a lens configuration of four lenses. For example, an imaging lens disclosed in Patent Reference includes a first lens that is positive and has a convex surface on the both sides; a second lens that is negative and has a shape of meniscus lens directing a convex surface to the object side; a third lens that is positive and has a shape of meniscus lens directing a concave surface to the object side; and a fourth lens that is negative and has a concave surface on the both sides arranged in this order from an object side. The configuration attains good optical performances while restraining increase of total length of the imaging lens by setting preferable range for a ratio between a focal length of the fourth lens and a focal length of the whole lens system and keeping the ratio within the range.
Patent Reference: Japanese Patent Application Publication No. 2009-20182
According to the imaging lens of Patent Reference, it is possible to obtain relatively good aberrations. However, miniaturization and functions of cellular phones are advanced every year, and cellular phones themselves are increasingly required to have a smaller size and higher functions. In the lens configuration disclosed in Patent Reference, it is difficult to attain both miniaturization and good aberration correction to satisfy those demands.
Furthermore, cameras mounted in cellular phones are used in various ways, and for example, those cameras are widely used to take picture of himself/herself with his/her friend(s) while holding his/her cellular phone with his/her hand, take picture of himself/herself with landscape as the background, etc. Since a person who takes a picture himself/herself is an object to take a photo in those uses, imaging lenses of cameras mounted in cellular phones are required to have enlarged imaging angle of view, i.e. wide angle of view.
Here, attaining both miniaturization and good aberration correction and wider angle is not specific issues to imaging lenses mounted in the cellular phones and is a common issue even in imaging lenses mounted in relatively small cameras such as digital still cameras, portable information terminals, security cameras, onboard cameras, and network cameras.
In view of the problems of the conventional techniques described above, an object of the present invention is to provide an imaging lens that has a small size, is capable of properly correcting aberration and has relatively wide imaging angle of view.